The present invention relates to display units and, more particularly, to an improvement in a display unit including an organic EL device.
A typical organic electroluminescent (EL) device has a composite structure including an organic EL layer sandwiched by electrodes, one electrode being composed of a metal such as aluminum, the other being a transparent electrode composed of, for example, indium tin oxide (ITO). A proportion of light emitted from the organic EL layer is directly output through the transparent electrode, while light output toward the metal electrode is reflected by the metal electrode and output through the transparent electrode. External light incident on the organic EL layer through the transparent electrode is also reflected by the metal electrode and then output through the transparent electrode. That is, the external light is added to the internal light emitted from the organic EL layer for forming an image. Thus, under external light, such as daylight, the contrast of the image decreases.
A display unit that prevents such a decrease in contrast is disclosed in Japanese Unexamined Patent Application Publication No. 9-127885. The display unit has circular polarization means, which is a combination of a linear polarizer and a quarter wave plate, at a light emitting face. Since the direction of circular polarization of the external light is opposite to the direction of the circular polarization reflected by the display unit, the external light is blocked by the circular polarization means. Japanese Unexamined Patent Application Publication No. 2000-315582 discloses an organic EL device having a visible light-absorbing layer between a cathode and an organic EL layer.
Color filters provided on red (R), green (G), and blue (B) light-emitting pixels and transmitting the R, G, and B light components, respectively, can suppress the reflection of external light other than the R, G, and B light components. However, these color filters cannot suppress the reflection of the R, G, and B light components of the external light. Accordingly, this organic EL device also has a low contrast. Furthermore, an organic EL display unit disclosed in Japanese Unexamined Patent Application Publication No. 2000-3786 has a green color with a main light transmission peak in the range of 490 to 530 nm. The green filter is disposed on a green organic luminescent layer.
These known technologies, however, have the following disadvantages:
(1) Although the display unit disclosed in Japanese Unexamined Patent Application Publication No. 9-127995 can effectively suppress the reflection of external light, the unit decreases the transmittance of light from the organic EL layer. Thus, the luminance is at most 50%.
(2) In the organic EL device disclosed in Japanese Unexamined Patent Application Publication No. 2000-315582, light from the organic EL layer is absorbed in the visible light-absorbing layer. Thus, the luminance is at most 50%.
(3) In a combination of R, G, B filters and corresponding R, G, B pixels, the reflectance of external light is about 30%. Thus, the external light is not effectively removed.